Anemometer Detecting Thermal Time Constant of Sensor

ABSTRACT

An anemometer and method for analyzing fluid flow is described. In one embodiment, a transistor sensor is heated by applying power to cause its base-emitter junction to rise from an ambient first temperature to a second temperature. The power is removed, and the Vbe is measured at intervals as the junction cools. The Vbe equates to a temperature of the junction. The temperature exponentially decreases, and the time constant of the decay corresponds to the fluid flow velocity. A best fit curve analysis is performed on the temperature decay curve, and the time constant of the exponential decay is derived by a data processor. A transfer function correlates the time constant to the fluid flow velocity. The transistor is thermally coupled to a metal rod heat sink extending from the package, and the characteristics of the rod are controlled to adjust the performance of the anemometer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on, and claims priority from, U.S. provisionalapplication 61/559,257, filed on Nov. 14, 2011, by the presentinventors, entitled Anemometer, assigned to the present assignee andincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to anemometers and, in particular, to a solidstate anemometer.

BACKGROUND

Anemometers measure the flow of a fluid, such as air, passing over it.In electrical cabinets or boxes containing heat producing circuitry, itis sometimes necessary to provide a fan for cooling. It is conventionalto provide instrumentation in the cabinet that monitors temperature andthe flow of air. The most common compact anemometers are hot-wires andthermocouples.

The hot-wire type comprises a wire that has a knownresistance-temperature characteristic. In one technique, the wire isheated by a current to achieve a target voltage across it and, as airflows over the wire to cool it, the current is controlled to maintain aconstant voltage. The amount of current therefore corresponds to the airvelocity once the anemometer is calibrated. Other techniques are alsoused, such as measuring the time it takes for the wire to cool afterbeing pulsed by a current. Such wires are expensive, are fragile,require frequent cleaning, and require calibration.

A thermocouple is the coupling between two dissimilar metals, where thejunction generates a temperature-dependent voltage. For use as ananemometer, as with the hot wire, the node may be heated by a current,and the voltage is measured. Like the hot wires, such thermocouples areexpensive, fragile, require frequent cleaning (if exposed), and requirecalibration.

Hot wires and thermocouples are essentially mechanical devices sincetheir characteristics are highly dependent on their mechanicalconstruction. It would be desirable to create an anemometer usingconventional integrated circuit fabrication techniques that is trulysolid state, so that the anemometer is inexpensive, rugged, and does notrequire calibration.

U.S. Pat. No. 3,968,685 discloses an anemometer where a base-emittervoltage drop of a bipolar transistor is compared to a voltage dropacross a reference (or compensation) diode. An adjustable current isprovided to keep the base-emitter voltage drop equal to the diodevoltage drop while the transistor is subjected to an air flow, and thiscurrent is equated to air velocity. The diode is use to compensate forambient temperature changes. It is difficult to prevent the diodevoltage drop from being affected by the air flow. Further, the designneeds precise calibration to cancel out the voltage drops at ambienttemperatures. Further, the transistor is thermally insulated by itsplastic package, making its base-emitter junction slow to respond tochanges in air velocity.

What is needed is a solid state anemometer that does not suffer from thedrawbacks mentioned above and related drawbacks.

SUMMARY

In one embodiment, an anemometer is disclosed that comprises a bipolartransistor packaged to have a metal pad thermally coupled to thetransistor. Such a metal pad is sometimes referred to as a thermal pad.The metal pad is electrically insulated from the transistor. The packagemay be mounted on a printed circuit board (PCB) so that its metal pad isfacing away from the PCB.

A metal rod is affixed to the thermal pad by solder or other thermallyconductive adhesive. In one embodiment, the metal rod is about 2-3 cmlong and less than 2 mm in diameter. The rod may be cylindrical foromnidirectional air flow measurement. The PCB also supports all or partof the control circuitry for the anemometer, whose operation is notsignificantly affected by temperature.

Power is applied to the transistor, creating a significantcollector-emitter current, which raises the base-emitter junctiontemperature and raises the temperature of the metal rod. The power isthen removed, and a small constant base current is applied to create abase-emitter voltage drop. The voltage drop corresponds in a knownmanner to the temperature of the junction, and this voltage drop issampled at precise intervals (e.g., at 100 ms intervals) for a period oftime (e.g., 1 minute). The readings are converted to digital signals byan analog-to-digital converter (ADC). All processing may be done on thePCB.

Junction temperature samples, derived from the voltage drop samples, arestored in a memory as the transistor cools via the flowing air removingheat from the metal rod. The cooling rate is an exponential decay. Therate of decay is substantially determined by the fixed thermalcapacitance of the package/rod and the variable thermal resistancebetween the junction and the air. The thermal resistance is lowered withincreased air flow since more rapidly flowing air creates an increasedheat flow from the junction to the air, resulting in more rapid coolingof the junction.

After the cooling cycle is completed (e.g., after 2 minutes), a best fitcurve analysis is performed on the temperature decay curve using, forexample, a least squares algorithm, and the time constant (TC) of thebest fit curve is calculated. The curve will have the general shape ofe^(−t/TC), where TC is the thermal time constant. When the time constantequals t, the temperature of the junction has fallen to about 37% of thefinal temperature. The time constant is determined by the air velocity.A lower time constant equates to an increased air velocity.

The derived time constant is then used in a transfer function to derivethe corresponding air flow velocity. The velocity may be communicated toa remote monitor, or may be used to generate a warning signal if it isoutside of an acceptable range.

The various calculations mentioned above may be condensed into fewercalculations, and the above description delineates the variousconversions as separate steps for clarity of explanation.

The circuitry is robust since the transistor is encased in a package.The circuit does not need calibration for adequate operation since itsconstruction is very repeatable. The circuit is much less expensive thananemometers using hot wires or thermocouples, and the results aresubstantially consistent from circuit to circuit and time to time.

Variations on the above-described technique and structures are alsodescribed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a printed circuit board on which ismounted the anemometer sensor and control circuitry.

FIG. 2 is a schematic diagram of one embodiment of the invention.

FIG. 3 is a flow chart of a process performed by the circuitry of FIG. 2in accordance with one embodiment of the invention to derive air flowvelocity.

FIG. 4 is a graph of the base-emitter junction temperature vs. time,showing the heating and cooling cycles and samples taken.

FIG. 5 is a graph of the base-emitter voltage drop (Vbe) vs. time as thesensor cools and the various measurements taken to derive air flowvelocity.

FIG. 6 is a graph illustrating how the derived thermal time constant ofthe best fit temperature curve equates to the flow rate of air.

FIG. 7 is a perspective view of the cylindrical metal rod heat sinkshown in FIG. 1 for omnidirectional air flow detection.

FIG. 8 is a perspective view of an elongated rectangular metal rod heatsink to provide directionality to the air flow sensing.

FIG. 9 illustrates a fan assembly including the anemometer.

Elements that are the same or equivalent are labeled with the samenumeral.

DETAILED DESCRIPTION

FIG. 1 illustrates a simplified printed circuit board (PCB) 10 on whichis mounted the anemometer sensor 12 and a controller 14. In oneembodiment, the PCB 10 is only 3×3 cm or less. The PCB 10 may beconventional and preferably thermally non-conductive. The circuitry onthe PCB 10 is connected by copper traces. The PCB's power and I/O pins16 may be inserted into a socket of a mother board or into a cableconnector. In one embodiment, it is desired to know the air flow atvarious locations in an electrical equipment box, and identical PCBs 10may be located at various locations in the box.

A bipolar transistor housed in a package 18 is mounted on the PCB 10.The package may be a surface mount package having three or moreterminals

The package 18 has a metal pad 20 to which the silicon die is thermallycoupled. The metal pad 20 is face up. A highly thermally conductive rod22 is affixed to the thermal pad 20 and acts as a heat sink. In oneembodiment, the rod 22 is copper and its shape is cylindrical foromnidirectional detection of air flow. In one embodiment, the rod 22 isless than 2 mm in diameter and about 2-3 cm long. The rod 22 mayterminate in a rectangular metal base that is either soldered orotherwise thermally coupled, such as with a thermally conductive epoxy,to the metal pad 20.

Slots 23 formed through the PCB 10 effectively thermally insulate thepackage 18 from other heat sources on the PCB 10.

In one embodiment, the package 18 is mounted over a thermally insulatingpad or an air gap to prevent the PCB 10 from acting as a heat sink forthe transistor. This will improve consistency in the air flow velocitymeasurements from system to system since the tolerances of the PCB 10and its connection to other heat sources and heat sinks will not affectthe anemometer. The wires connecting the transistor to the PCB 10terminals should be as thin as possible to minimize the transfer of heatbetween the transistor and the PCB 10.

An optional temperature sensor 24 is also mounted on the PCB 10 fordetermining the temperature of the PCB 10. The ambient air temperaturemay be optionally detected by the sensor 12 by measuring thebase-emitter voltage drop while the sensor 12 is not being used fordetecting air flow velocity.

A fan 30 draws cool air 31, through a filter, from outside the box, andthe heated air escapes though vents in the box. The sensor 12 may beplaced directly in front of the fan 30 to detect the air flow generatedby the fan 30 or may be located in other locations in the box to detectif there is adequate air circulation.

Cooling efficiency is the ability to remove heat from an object (e.g., atransistor) and is a function of air flow over it, air temperature,object temperature, humidity, barometric pressure, surface contamination(dust), and other factors. For short periods of time, all factors exceptthe object temperature may be assumed to be constant and the air flowrate may be suitably derived using the techniques described herein. Formore accurate measurement, the PCB 10 temperature may be taken intoaccount as well as any detected air temperature transients.

Since the bipolar transistor die, package 18, and rod 22 may befabricated to strict tolerances and specifications (the packagedtransistor must meet its data sheet specifications), their performancecan be very predictable without any calibration by the user.

FIG. 2 is a schematic diagram of one embodiment of the invention, whichmay be entirely contained on the PCB 10. The various switches in FIG. 2may be transistor switches.

FIG. 3 is a flow chart of a process performed by the circuitry of FIG. 2in accordance with one embodiment of the invention to derive air flowvelocity. In an actual embodiment, the various algorithms and stepsidentified in FIG. 3 may be condensed into fewer algorithms to saveprocessing time, but the algorithms are delineated in FIG. 3 for clarityof explanation.

In step 40 of FIG. 3, the switches 42 and 44 are closed to drive thetransistor 46 at a certain current for a certain time to heat it. Thetransistor 46 may be any bipolar transistor in a suitable package with ametal thermal pad, available from a variety of manufacturers. Areference voltage source 47 provides a fixed reference voltage to oneinput of a differential amplifier 48, and its other input is the voltageat the emitter resistor 50. The current through the transistor 46 is setso that the voltage at the resistor 50 equals the reference voltage. Theoptimal current is dependent on the type of transistor 46 used and ispreferably relatively high to quickly heat the transistor, such as 80%of its rated current. The amount of heating is determined by the currenttimes the voltage across the transistor 46 (between nodes 52 and 53).This power is dissipated by the transistor 46 during the heating cycle.A timer 54, which includes a clock and a state machine, controls thetiming of the various switches and the processing.

In step 58, the transistor 46 temperature rises to a peak temperature,determined by the power and the timer 54. The temperature does not haveto level off, and the absolute temperatures are not relevant to thedetermination of air flow velocity. FIG. 4 illustrates the temperaturerise of the base-emitter junction during the heating cycle from timeT0-T1.

At time T1, the timer 54 opens switches 42 and 44 and closes switches 60and 62, causing the base and collector to be shorted and the currentfrom the current source 64 to flow through the base-emitter junction tocreate a Vbe diode drop. The Vbe is dependent on temperature in a wellknown manner.

The standard equation for current through a diode is:

I=I _(S)*(exp(V/(n*k*T/q))−1)  Eq. 1

Where:

I is the current through the diode

I_(S) is the reverse saturation current

V is the voltage across the diode

n is a junction constant (typically around 2 for diodes, 1 fortransistors, determined empirically)

k is Boltzmann's constant, 1.38E-23 Joules/Kelvin

T is temperature in Kelvin, and

q is the magnitude of an electron charge, 1.609E-19 coulombs.

The sub-expression, k*T/q, has units of voltage and is referred to asthe thermal voltage VT. VT is typically around 26 millivolts at roomtemperature.

Equation 1 can be solved for the forward voltage as:

V=n*VT*ln [(I/I _(S))+1]  Eq. 2

Accordingly, given that the current I of the current source 64 is known,the diode drop V (i.e., Vbe) is known by measurement, and the constantsare known for the particular transistor (e.g., provided by thetransistor manufacture or otherwise determined empirically), the onlyunknown is the junction temperature T, which is easily calculated.

In step 66 of FIG. 3, the Vbe is measured by the timer 54 controllingthe analog-to-digital converter (ADC) 68 to sample the Vbe at certainintervals, such as at 100 ms intervals. The ADC 68 may have anyresolution, such as from 14 to 24 bits, depending on the accuracydesired. The emitter voltage is equal to the current source 64 currenttimes the resistor 50 value, and the base voltage is detected via theclosed switch 62.

In step 70, the ADC 68 converts the Vbe measurements to digital codesfor digital processing by the data processing circuit 72. FIG. 5illustrates an example of the Vbe values 73 measures during the coolingcycle over 20 seconds, where the Vbe had an initial value of about 0.6volts at the beginning of the cooling cycle and a voltage of about 0.7volts at 20 seconds.

In step 74, the data processing circuit 72 uses Equation 2 to calculatethe junction temperature associated with each Vbe measurement. Thesetemperatures are then stored in a memory as part of the data processingcircuit 72, and the temperature value set defines an exponential decaycurve. FIG. 4 illustrates temperature values 76 derived from the Vbesamples and stored in the memory. The data processing circuit 72 may usefirmware to perform all calculations or may use a programmedmicroprocessor. As stated previously, the various conversions describedherein may be condensed into fewer steps, and the steps in FIG. 3 aredelineated for clarity of explanation. For example, the conversion fromthe Vbe data to the temperature data need not be performed, and aconversion algorithm (a transfer function) is used to convert the timeconstant of the Vbe decay directly to the air flow velocity.

In step 78, after a predetermined period (e.g., 2 minutes) or after itis determined that sufficient cooling has taken place (e.g., the deltatemperature or Vbe values are approximately zero), the data processingcircuit 72 then applies a well-known algorithm, such as a least squaresalgorithm, to the temperature data to create a best fit curve having theexponential decay properties of e^(−t/TC), where TC is the thermalconstant. Any other constants moving the entire curve up or down may beignored since only the rate of decay is relevant.

When TC equals t, the junction temperature has dropped to about 37% ofthe peak junction temperature. In another embodiment, the curve may alsoinclude linear equations or polynomial equations that affect thecalculation of the time constant. In another embodiment, the best fitcurve analysis is performed directly on the Vbe curve, obviating anyneed to calculate the temperatures associated with each Vbe measurement.

In step 80, the data processing circuit 72 calculates the thermal timeconstant TC from the best fit curve. TC may be the time constant of thetemperature curve or the Vbe curve.

In step 82, the data processing circuit 72 then uses a transfer functionor look-up table to equate the TC to an air flow velocity. The transferfunction is an algorithm that is created using experimental data tocorrelate the time constant to the air flow velocity. Different transferfunctions may be used for different applications or air characteristics.In one embodiment, the transfer function is a fourth order transferfunction. In another embodiment, a look-up table may be used tocross-reference the time constant to the air flow velocity. The transferfunction or look-up table may generate the values shown in the TC vs.air flow graph of FIG. 6, where air flow is given in linear feet perminute. In the example, the time constant was calculated to be about 11seconds, corresponding to an air flow velocity of 187 linear feet perminute. Any units may be used, such as miles per hour, etc. The graph ofFIG. 6 (in the form of a transfer function or look-up table) may begenerated empirically by the anemometer manufacturer during testing of asample anemometer in an air flow chamber. The TC may be that of thetemperature decay curve or the Vbe decay curve.

There may be a variety of transfer functions or look-up tables stored inthe system for converting the TC to the air flow velocity, where theselection of the particular function or look-up table depends onmeasured ambient air temperature (air intake temperature), humidity,barometric pressure, etc., where such variables affect the rate ofcooling of the base-emitter junction to varying degrees. For example,higher humidity, higher barometric pressure, or higher ambienttemperature may each cause an air flow to cool the junction at adifferent rate. Such detectors may be remote from the PCB 10 (FIG. 1)and feed data to the data processing circuit 72 via the I/O pins 16.Such fine tuning of the air flow velocity measurement is not necessaryin many applications where a precise measurement is not required.

The temperature of the PCB 10 (FIG. 1) may also affect the TC vs. airflow values if there is thermal coupling between the PCB 10 and thetransistor 46 and the PCB 10 temperature fluctuates during a measurementcycle. The temperature sensor 24 may supply such PCB 10 temperature datato the data processing circuit 72 to correct for such transients.

Once the air flow velocity is determined, it may be output via the I/Opins 16 to an external monitor, or it may be compared to a pass/failthreshold programmed into the data processing circuit 72. The dataprocessing circuit 72 may issue a warning signal if the air flowvelocity is determined to be outside the acceptable range, as shown instep 84.

Other related techniques are also envisioned where the thermal timeconstant is derived to determine the air flow velocity.

Although the calculations have been delineated in FIG. 3 for clarity ofexplanation, the Vbe value set may be applied directly to an algorithmthat derives the thermal time constant and the air flow velocity withoutthe separate steps of calculating junction temperatures, etc. Suchcondensing of the algorithms is within the skills of those in the fieldof programming.

In another embodiment, the following expression may be used to derive adecay constant between any two Vbe samples, and a least squarestechnique is used to derive the best fit decay constant to the curve forthe entire set of Vbe samples. The best fit decay constant then is usedto determine the air flow velocity using a suitable look-up table.Hence, a time constant, related to a rate of decay of either Vbe ortemperature, is used for deriving the air flow velocity.

Vbe(n+M)=Vbe(n)*(decay constant)^(M)+constant2  Eq. 4

Where, n is a sample (e.g., 1, 2, 3, etc.) and

n+M is the n+M sample (e.g., 2, 3, 4, etc.).

Other techniques may be used to measure the air flow using the basicanemometer structure of FIG. 1.

In one embodiment, a resistive heater is mounted on the PCB 10 next tothe package 18, in the package 18, or under the package 18, and acurrent is applied to the heater to heat the package 18 during theheating cycle (FIG. 4) of the base-emitter junction.

In another embodiment, the package 18 may be initially cooled by, forexample, a Peltier cooler located under the package 18. The junction iscooled for a period to create an initial temperature, then power to thePeltier cooler is removed. The rate of heating of the junction toambient temperature, or any other temperature, is then related to theair flow velocity, and such velocity is calculated using techniquessimilar to those described above.

In another embodiment, the Vbe is measured at two known currents atperiodic intervals during the cooling cycle. The temperature of thejunction may then be calculated at each interval as:

T=[q/(n*k)]*[Vbe2−Vbe1]/ln(Id2/Id1)  Eq. 3

Where Vbe1 and Vbe2 are the base-emitter voltage drops at the currentsId1 and Id2, respectively.

The best fit exponential decay temperature curve is then calculated, andthe thermal time constant TC (or the Vbe time constant) is then derivedto determine the air flow velocity, as previously described.

In another technique, a first known power level is applied to thetransistor 46 by controlling the voltage source 48 or resistor 50. Aftera certain delay to ensure the temperature has settled, the temperatureof the base-emitter junction is then derived. The power is thenincreased to a second known current level, and the junction temperatureis measured again after a certain time. The equation delta P/delta Tthen conveys the thermal resistance between the junction and the airwhile cancelling out any constants. The thermal resistance is related tothe air flow velocity in a known manner, so air flow velocity may thenbe calculated. For example, for little or no air flow, an increasedpower will result in a large increase in junction temperature, while ahigh air flow will result in a lower increase in junction temperature.The change in temperatures can thus be cross-referenced to air flowvelocity by using a transfer function or look-up table. This alternativetechnique is considered not as reliable as the technique of FIG. 3 wherethe thermal time constant is used to derive air flow velocity, sincethis alternative technique is more sensitive to temperature transientsduring the test time due to the longer settling time requirement. In arelated embodiment, a suitable transfer function is directly applied tothe changes in Vbe to derive the air flow velocity, where the transferfunction takes into account the relationship between Vbe and thejunction temperature.

The temperature curve analysis algorithm, or alternatively a Vbe curveanalysis algorithm, may be designed to identify anomalous measurementsindicative of thermal or air flow transients. If such anomalies aredetected, the program disqualifies the measurement cycles, and thedetection process is repeated.

In valuable equipment systems without an anemometer, periodicreplacements of the air filters are performed in the event that they areblocked by dust or dirt. By using the present anemometer, there is noneed to automatically replace the air filters at predefined intervalssince it is assumed the filter is adequate if the air flow velocity iswithin the acceptable range. The proper operation of the fans is alsodetermined by the anemometer.

Additionally, during the design of the equipment box (e.g., a server,slot machine, etc.), several of the anemometers may be distributedthroughout the box where circuitry is located to determine if the airflow is adequate around that location. If not, the fan or circuitrylocation may be changed to achieve adequate cooling. In the finalproduct, only one anemometer may need to be used proximate to the fan orair exit vent to determine the proper operation of the cooling system.However, valuable system data may be gathered from the field by leavinga full complement of anemometers in the final product.

Although air has been used as the fluid in the examples, the anemometermay be used to measure the flow velocity of any fluid, such as a liquid.

In addition to the PCB 10 circuitry providing the air flow velocity, thetemperature sensor 24 or the base-emitter voltage drop of the transistor46 may be used to convey ambient temperature between the air flowvelocity measurement cycles, assuming the transistor temperature hassettled. This ambient temperature may be transmitted to a remote monitorvia the I/O pins 16, or a warning signal may be generated if thetemperature is outside an acceptable range.

As a premise for accurate air (fluid) flow velocity measurement withoutrequiring calibration, the anemometers should not significantly varyfrom one anemometer to the next. The transistor 46 specifications areadequately controlled by the manufacturer and specified in a data sheet.The precision of the other circuitry must also be adequately controlledand not significantly affected by ambient temperature. Any variation incircuit performance due to ambient (PCB 10) temperature variation may becorrected by measuring the temperature of the PCB 10 and making thecorresponding corrections to the data. However, since the ambienttemperature is assumed to change at a slow rate relative to the coolingperiod for the air flow velocity measurement, the correction due tochanges in ambient temperature is considered optional for manyapplications.

Although the size and construction of the package 18 may vary due tomanufacturing tolerances, the effects of such variations on cooling ofthe transistor 46 may be minimized by carefully controlling thedimensions and mass of the metal rod 22 heat sink. In other words, thecharacteristics of the metal rod 22 heat sink may be controlled todominate the properties of the anemometer.

Since the dimensions of the metal rod 22 heat sink significantly affectthe performance of the anemometer, the dimensions of the metal rod 22should be carefully controlled to avoid the need for calibration. Inexperiments performed by the inventors, a 1% variation of the derivedthermal time constant TC (under identical conditions) from oneanemometer to the next results in about a 10% variation in the linearfeet per minute air flow velocity calculation, which is significant.

A particularly important characteristic of the metal rod 22 is its mass,directly related to its volume. A variation in its diameter has beenshown to be more significant than the same percent change in its height,since variations of the thermal mass of the rod 22 are cancelled to somedegree by related variations in the surface area of the rod 22. Thethermal time constant of the rod (thermal TC) is a function of the massof the rod 22 divided by the surface area of the rod 22. Accordingly,the diameter of the rod 22 must be tightly controlled since it has themost effect on the thermal TC of the rod 22. In one example, a diametertolerance of +/−0.25 mil and a height tolerance of +/−1 mil results in a+/−3.1% tolerance in the air flow velocity measurement, which isacceptable.

Since the mass of the rod 22 is much greater than the mass of the metalpad 20 of the package 18, variations in the thermal pad 20 are not verysignificant. The tolerances of the rod 22 can be more tightly controlledthan the tolerances of the metal pad 20.

FIG. 7 shows the cylindrical rod 22 of FIG. 1 having a metal base 90 forsoldering to the metal pad 20 (FIG. 1) of the package 18. The shape ofthe rod 22 may be varied for directionality, since the effective surfacearea exposed to the air flow can be directionally controlled. Forexample, as shown in FIG. 8, the rod 92 may be a rectangular column toincrease its surface area exposed to the air flow if the wider flatsurface of the rod 92 were angled normal to the direction of the airflow. Multiple rods 22/92 may be attached to a single base 90, such asone rod per corner of the base 90, where the orientation of the rodarray changes the surface area exposed to the air flow.

If the tolerances of the metal rod 22 heat sinks are not verycontrollable (e.g., greater than 1%), then calibration of the anemometermay be needed if high accuracy of the air flow velocity determination isrequired. Calibration may be done with zero air flow velocity. However,for a pass/fail air flow detector, no calibration is needed.

Since the temperature of a sensor with a larger thermal TC generallyreacts more slowly to changes in air flow, the mass of the rod 22 (andto a lesser extent, its surface area) affects the ability of theanemometer to respond to air flow transients. So the rod 22 may bespecifically designed for a particular application to control theanemometer's sensitivity to air flow transients. In contrast to a hotwire, the thermal TC of a hot wire is very small, so the wire is verysensitive to air flow transients, requiring electronic averagingtechniques to derive the average air flow velocity.

In one embodiment, instead of a transistor 46 being used as thetemperature sensor, a diode or resistor may be used if it can besufficiently heated by a high current during the heating cycle.

FIG. 9 illustrates a fan assembly 96 including the anemometer. A fan 98has a frame that is molded to provide a support arm 100 for the PCB 10and the anemometer. The rod 22 heat sink is shown. A cable that suppliespower and control to the fan 98 also electrically connects to the pins16 (FIG. 1) on the PCB 10. Thus, the PCB 10 is thermally insulated fromother equipment and optimally located in front of the fan 98 to detectany obstruction of air at the air intake port or any malfunction of thefan. The anemometer algorithms and look-up tables may be calibrated forthe fan assembly 96, resulting in extremely accurate air flow velocitymeasurements.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects. The appended claims are to encompasswithin their scope all such changes and modifications as fall within thetrue spirit and scope of this invention.

What is claimed is:
 1. A method for analyzing fluid flow comprising: a.applying power to cause a temperature of a diode junction to change froma first temperature to a second temperature; b. changing the power tocause the temperature of the diode junction to change from the secondtemperature toward the first temperature at a substantially exponentialrate, the rate corresponding to a time constant, where the time constantis related to a fluid flow interacting with the diode junction; c.measuring a changing voltage drop across the diode junction at varioustimes as the temperature changes from the second temperature toward thefirst temperature; and d. correlating the time constant to fluid flow.2. The method of claim 1 wherein the diode junction is a base-emitterjunction of a bipolar transistor.
 3. The method of claim 1 wherein stepa comprises applying a current through a semiconductor device comprisingthe diode junction to heat the diode junction to the second temperature.4. The method of claim 1 wherein step a comprises applying a current toa heater device thermally coupled to the diode junction to heat thediode junction to the second temperature.
 5. The method of claim 1wherein step d comprises performing a best fit curve analysis tocalculate the rate of change of the temperature.
 6. The method of claim1 wherein a time constant of the changing voltage drop corresponds tothe time constant of the change from the second temperature to the firsttemperature, and wherein step d comprises correlating the time constantof the changing voltage drop to fluid velocity.
 7. The method of claim 1wherein step d comprises correlating the rate of change of the voltagedrop to cooling efficiency.
 8. The method of claim 1 wherein step dcomprises applying a transfer function to a time constant of thechanging voltage drop.
 9. The method of claim 1 wherein step d comprisesapplying a transfer function to a time constant of the changingtemperature.
 10. The method of claim 1 wherein a heat sink is thermallycoupled to the diode junction, the method further comprising flowing thefluid across the heat sink to remove heat from the diode junction. 11.The method of claim 10 further comprising trimming a mass of the heatsink, without significantly changing a surface area of the heat sink, tocontrol a ratio of mass vs. surface area of the heat sink.
 12. Themethod of claim 1 further comprising the step of analyzing the change inthe voltage drop to determine whether any variations in the rate ofchange of the voltage drop are above a threshold, indicating fluid flowtransients, and, if so, disqualifying the time constant of thetemperature change as an accurate indication of fluid flow.
 13. Themethod of claim 1 wherein the diode junction is in a package and thepackage is mounted on a printed circuit board (PCB), the method furthercomprising: sensing a temperature of the PCB; detecting transients inthe temperature of the PCB; and correcting a fluid flow calculation instep d using data related to the transients in the temperature of thePCB.
 14. The method of claim 13 wherein the step of sensing thetemperature of the PCB comprising detecting a voltage drop across thediode junction.
 15. The method of claim 1 wherein the diode junction isin a package and the package is mounted on a printed circuit board(PCB), the method further comprising: sensing a temperature of the fluidusing a temperature sensor with a thermal time constant matched to athermal time constant of the diode junction; detecting transients in thetemperature of the fluid using the temperature sensor; and correcting afluid flow calculation in step d using data related to the transients inthe temperature of the fluid.
 16. An anemometer comprising: a. a diodejunction mounted in a package, the package comprising a metal thermalpad electrically insulated from the diode junction; b. at least one heatsink in thermal contact with the thermal pad and extending away from thethermal pad for flowing a fluid across the heat sink to remove heat fromthe diode junction; c. at least one temperature control switch coupledto a power supply; d. a controller for controlling the at least onetemperature control switch for an interval to change a temperature ofthe diode junction from a first temperature to a second temperature,then allow the temperature of the diode junction to change from thesecond temperature toward the first temperature at a substantiallyexponential rate, the rate corresponding to a time constant, where thetime constant is related to a fluid flow interacting with the diodejunction; e. a digital processor configured to measure a changingvoltage drop across the diode junction at various times as thetemperature changes from the second temperature toward the firsttemperature; and f. the digital processor being configured to correlatethe time constant to fluid flow.
 17. The anemometer of claim 16 whereinthe diode junction is a base-emitter junction of a bipolar transistor.18. The anemometer of claim 16 wherein the diode junction is abase-emitter junction of a bipolar transistor and wherein the at leastone temperature control switch is coupled between the power supply and acollector of the transistor to supply a current through the transistorto heat the transistor to the second temperature.
 19. The anemometer ofclaim 18 wherein the at least one temperature control switch comprises:a first switch coupled between the power supply and the collector of thetransistor to supply a first current through the transistor to heat thetransistor to the second temperature; and a second switch coupledbetween a base of the transistor and a base voltage source for drivingthe transistor to conduct the first current between the collector and anemitter of the transistor.
 20. The anemometer of claim 16 furthercomprising at least one measurement control switch controlled by thecontroller to drive the diode junction at a current while coupling thevoltage drop across the diode to an analog-to-digital converter (ADC),an output of the ADC being coupled to the digital processor.
 21. Theanemometer of claim 16 wherein the digital processor is configured toperform a best fit curve analysis to calculate the time constant of thetemperature change.
 22. The anemometer of claim 16 wherein the at leastone temperature control switch is coupled between the power supply and aheater device thermally coupled to the diode junction to heat the diodejunction to the second temperature.
 23. The anemometer of claim 16wherein the digital processor is configured to calculate the timeconstant of the temperature change by determining a rate of change ofthe voltage drop and correlating the rate of change of the voltage dropto fluid velocity.
 24. The anemometer of claim 16 wherein the digitalprocessor is configured to apply a transfer function to a time constantof the changing voltage drop to derive the fluid flow.
 25. Theanemometer of claim 16 wherein the digital processor is configured toapply a transfer function to a time constant of the changing temperatureto derive the fluid flow.
 26. The anemometer of claim 16 wherein theheat sink comprises at least one rod extending from the thermal pad. 27.The anemometer of claim 26 wherein the rod is cylindrical.
 28. Theanemometer of claim 26 wherein the rod is a polyhedron.
 29. Theanemometer of claim 26 wherein a length of the rod is greater than 1 cm.30. The anemometer of claim 16 wherein the digital processor isconfigured to analyze the change in the voltage drop to determinewhether any variations in the rate of change of the voltage drop areabove a threshold, indicating fluid flow transients, and, if so,disqualifying the time constant of the temperature change as an accurateindication of fluid flow.
 31. The anemometer of claim 16 wherein thediode junction package is mounted on a printed circuit board (PCB), theanemometer further comprising a temperature sensor for sensing atemperature of the PCB, the digital processor being configured to detecttransients in the temperature of the PCB using the temperature sensorand correct a fluid flow calculation using data related to thetransients in the temperature of the PCB.
 32. The anemometer of claim 16further comprising a temperature sensor separate from the diode junctionthat detects a temperature of the fluid, a thermal time constant of thetemperature sensor being matched to a thermal time constant of the diodejunction, the digital processor being configured for detectingtransients in the temperature of the fluid using the temperature sensorand correcting a fluid flow calculation using data related to thetransients in the temperature of the fluid.
 33. The anemometer of claim16 wherein the diode junction package is mounted on a printed circuitboard (PCB), the PCB having slots at least partially surrounding thepackage to increase thermal isolation between the PCB and the diodejunction.
 34. The anemometer of claim 16 further comprising a supportmember forming part of a fan module so as to position the heat sinkdirectly in an air flow path in front of the fan.
 35. An anemometersystem comprising: a temperature sensor having electricalcharacteristics that vary with temperature; a heat sink thermallycoupled to the sensor, the heat sink comprising an elongated metal rodextending from the sensor, the rod having a mass; the rod being locatedin a fluid flow for removing heat from the rod; and a controllerconnected to the sensor for detecting the electrical characteristics ofthe sensor as heat is removed from the rod.
 36. The anemometer of claim35 wherein the rod has a length dimension, wherein the length dimensionis substantially normal to a primary direction of fluid flow.
 37. Theanemometer of claim 35 wherein the mass of the rod is adjusted to selecta response of the anemometer to transients in the fluid flow.